1) There have been plenty of apps built using the Google Maps and GMail methodologies. The first app I built using remote scripting functionalities similar to Gmail was in 2001, and relatively speaking, I was a latecomer. I found out about the technologies through simple web searching. Here, for example, is an Apple Developer connection article from a few years ago that I still have bookmarked:

developer.apple.com/internet...rame.html

I think this article dates back to 2002, and there are similar, even older articles on the O'Reilly site.

2) Until recently, most work using these technologies has been in the corporate, behind-the-firewall, ASP(App Service Provider)-hosted, etc apps. This is because, while it's possible to build apps like this that work in Netscape 4, it's a pain in the ass. Corporate web-apps generally have far more controllable browser requirements. Microsoft's original remote-scripting technology worked cross-browser, but only if the server side was ASP(Active Server Pages). Ashley IT developed a bunch of ways around this, and I know these guys date back to 2001 at the latest:

www.ashleyit.com/rs/main.htm

3) The reason you're starting to see this technology used now in publicly accessable websites is largely thanks to standards advocates like Zeldman and WASP - Big companies like Google are finally dropping support for Netscape 4, which makes building apps like this a whole lot easier.